For me it depends on price, if it's a sale price and I absolutely love-love it then I'll get up to 5 yds of each fabric in the collection that I love. Most of my quilts go to charity so I tend not to look at the non-sale fabrics. I'd go bankrupt. 5 yds is roughly what I need for backing. So I know I have enough to do something with.
I had bought just 1 yard or a fat quarter of something I loved in the past, but I've found that sometimes it's hard to find good coordinating fabric a few years later. Or it's not enough for my idea.